Conventionally, various starters for cranking an internal combustion engine have been known. Typically, in such a starter, as disclosed in Japanese patent laid open publication No. 59-221463, and U.S. Pat. Nos. 4,604,907, 4,561,316, 4,573,364, 4,520,285, 4,510,406, and 4,528,470, a pinion is selectively meshed with a ring gear or an internal gear of the engine to crank the same according to the on-off action of an electromagnetic switch by way of a one-way clutch or an overrunning clutch which is driven by a DC motor via a planetary gear reduction unit.
Since the starter includes various mechanical components--such as a spline coupling portion, an overrunning clutch and a reduction gear unit--which are sensitive to moisture and contamination, the starter is desired to be equipped with structures for keeping off water and dust. In the conventional starter, the drive shaft and the input member of the overrunning clutch are coupled to each other via a spline coupling portion, and the overrunning clutch can reciprocate axially according to the on-off action of the starter while transmitting rotational torque from the reduction gear unit to a drive pinion. The rest position of the overrunning clutch when the starter is not operating is typically defined by the abutment of an end portion of the spline groove on the drive shaft with an axial end of the spline teeth of the input member of the overrunning clutch, and this abutting engagement of the two parts is not capable of effectively shutting off water and dust. Therefore, there was a possibility of intrusion of water and dust into the spline coupling portion from the gaps of the spline teeth.
U.S. Pat. No. 4,561,316 discloses an annular end surface of the input member of its overrunning clutch abutting an end surface of a reduction gear unit by way of a brake washer 58. However, there is no mentioning of the capability of the brake washer to function as a sealing element. Furthermore, when the starter is activated, the two parts are quickly moved away from each other, and water and other foreign matters can easily pass through the gap between these two parts.
Further, since it is not always possible to keep water completely out of the housing accommodating the internal gear, the casing of the starter which communicates with the interior of the housing is provided with a water draining hole which faces outside of the engine when the starter is mounted on the engine. Also, in the conventional starter, the mounting flange is provided in a base end of its cylindrical mounting portion, and the water draining hole opens externally of the mounting surface of the engine associated with the mounting flange. Therefore, water splashed up by the vehicle during its motion may enter the interior of the starter through this water draining hole.
In such a starter, the pinion is integrally formed in the output member of the overrunning clutch constructed as a clutch inner member, and, if the water which, for instance, may have reached the interior of the starter from the side of the ring gear of the engine adheres to the pinion or a part of the clutch output member adjacent the clutch input member, there is a chance that the water may enter the interior of the clutch from a gap defined between a clutch cover integrally attached to the clutch outer member and the clutch inner member. Thus, such an intrusion of water should be avoided as much as possible.